1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device and particularly to improvement of its luminous efficacy.
2. Description of the Background Art
In a nitride semiconductor light-emitting device such as a nitride semiconductor LED (Light Emitting Diode), according to Japanese Patent Laying-Open No. 06-268257, an active layer interposed between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer has a multiple quantum well (MQW) structure in which a quantum well layer of In0.2Ga0.8N and a barrier layer of In0.04Ga0.96N are repeated alternately for 5 periods.
Further, according to Japanese Patent Laying-Open No. 11-330554, in an MQW active layer of a nitride semiconductor light-emitting device, a well layer of In0.4Ga0.6N and a barrier layer of GaN are repeated alternately for 5 periods.
As described above, in the prior art according to Japanese Patent Laying-Open Nos. 06-268257 and 11-330554, each barrier layer is formed of a single layer of GaN or InGaN.
Incidentally, since InN has a smaller energy band gap compared to GaN, a layer having relatively high In concentration can act as a quantum well layer, while a layer having relatively low In concentration can act as a barrier layer. From a viewpoint of improving luminous efficacy by trapping and recombining carriers in the well layer of the MQW active layer, it is preferable that there is larger difference in band gap between the quantum well layer and the barrier layer, and it is also preferable that the thickness of the barrier layer can be increased to some extent.
As a factor of improving the luminous efficacy by enhancing the quality of MQW active layer in the nitride semiconductor light-emitting device, it may be effective to reduce influence of distortion caused at the interface between the well layer and the barrier layer by lattice mismatch between these layers.
When GaN is used for the barrier layer as described in Japanese Patent Laying-Open No. 11-330554, it is difficult to avoid the influence of distortion caused by lattice mismatch between the InGaN well layer and the GaN barrier layer. Particularly, when a well layer having a high In concentration or a barrier layer having a large thickness is desired in an MQW active layer in a nitride semiconductor light-emitting device for emitting blue light or green light, the influence of distortion caused by lattice mismatch becomes more noticeable.
When InGaN is used both for the well layer and the barrier layer as in Japanese Patent Laying-Open No. 06-268257, it is not possible to increase In concentration in the barrier layer that is required to have sufficiently higher band gap compared to the well layer. Specifically, the In concentration must be made different between the well layer and the barrier layer, and thus it is difficult to avoid the influence of distortion caused by lattice mismatch between the well layer and the GaN barrier layer.